This invention relates to optical transmitters for use in communications systems, and to anamorphic lenses for use in such transmitters.
The bandwidth explosion driven by Internet usage has created a demand for an adaptable, high performance optical network. One of the key technologies in delivering high performance over a long haul network is pump laser technology, particularly Raman Laser technology. From a market perspective, device output power is the most important performance characteristic of such devices. There is considerable commercial pressure to maximise the overall device output power and efficiency. It is of course an advantage to operate at high laser powers as this can increase the spacing between amplifier stations in an amplified system and thus provide a significant cost reduction. Unfortunately, high power laser sources tend to produce highly elliptical beam and highly divergent shapes which, for the reasons discussed below, substantially reduce the coupling efficiency and hence the effective device output power. In particular, it has been found that the high power lasers now becoming available have such a high degree of beam divergence and ellipticity that this cannot be fully corrected by the use of conventional anamorphic lenses.
A typical optical transmitter comprises a laser capable of producing a divergent beam, and a lens whereby that divergent beam is collimated into a parallel beam for launch into an optical wave guide. In general, the laser output comprises an elliptical beam, and it has been found that this ellipticity increases as higher power laser are employed. This is a particular problem as high beam ellipticity substantially reduces the coupling efficiency of the laser output into a circular fibre. Lack of overlap between the elliptical laser beam and the fibre mode reduces the coupling efficiency. This effect is illustrated in FIGS. 1a to 1c of the accompanying drawings. FIG. 1a is a cross-sectional view of an elliptical laser beam 10. FIG. 1b is a cross-sectional view of a typical optical fibre core 12 which is substantially circular in cross section. FIG. 1c shows the mismatch effect in which the lobes 10a, 1b, fail to enter the fibre or waveguide 12 and thus represent lost energy. If for example the ellipticity of the laser beam is e, then the maximum theoretical coupling efficiency into a circular cross section fibre is given by the expression:   MaxEfficiency  =            4      ⁢      e                      [                  1          +          e                ]            2      
For a typical laser beam ellipticity of about 3.0, the coupling efficiency is thus reduced to about 75%, which translates into a loss of 1.25 dB without taking into account other losses. Since, for most pump laser products, output power is an exceptionally strong market driver, there is great incentive to solve this problem. To correct this loss of efficiency resulting from beam ellipticity, a number of workers have employed anamorphic lenses in conjunction with the laser. In a conventional anamorphic lens design, the two lens surfaces are crossed toroidal surfaces. An example of an anamorphic lens might for example be crossed cylinders, each of appropriate radius of curvature. One surface may comprise a cylindrical lens surface that collimates the laser beam in one plane and the second surface is a cylindrical lens that collimates the beam in the orthogonal plane. A general expression for the surface contour of each surface of such a lens is typically a polynomial expression of the form:                                                         2              )                        ⁢                          xe2x80x83                        ⁢            z                    =                                                    y                2                                                              R                  y                                +                                                                            R                      y                      2                                        -                                                                  (                                                  1                          +                                                      k                            y                                                                          )                                            ⁢                                              xe2x80x83                                            ⁢                                              y                        2                                                                                                                  +                                          α                1                            ⁢                              xe2x80x83                            ⁢                              y                2                                      +                                          α                2                            ⁢                              xe2x80x83                            ⁢                              y                4                                      +                                          α                3                            ⁢                              xe2x80x83                            ⁢                              y                6                                      +                                          α                4                            ⁢                              xe2x80x83                            ⁢                              y                8                                      +                                          α                5                            ⁢                              xe2x80x83                            ⁢                              y                10                                      +                                          α                6                            ⁢                              xe2x80x83                            ⁢                              y                12                                      +            …                          ⁢                  
                ⁢        3            )        ⁢                  xe2x80x83            ⁢              xe2x80x83              ⁢    z    =                    x        2                              R          x                +                                            R              x              2                        -                                          (                                  1                  +                                      k                    x                                                  )                            ⁢                              xe2x80x83                            ⁢                              x                2                                                          +                  α        1            ⁢              xe2x80x83            ⁢              x        2              +                  α        2            ⁢              xe2x80x83            ⁢              x        4              +                  α        3            ⁢              xe2x80x83            ⁢              x        6              +                  α        4            ⁢              xe2x80x83            ⁢              x        8              +                  α        5            ⁢              xe2x80x83            ⁢              x        10              +                  α        6            ⁢              xe2x80x83            ⁢              x        12              +    …  
The first term on the left of each expression represents the curvature of the respective lens surface, and the remaining polynomial terms provide aberration correction. However, it has been found that the above designs only provide effective aberration correction along the x and y axes. The even aspheric surfaces set out in equations 2) and 3) provide aberration correction using a respective conic term, kx or ky1, plus even polynomial terms, a sufficient number of terms being taken to achieve the desired accuracy. For the sake of simplicity, one can regard the quadratic terms as supplying the lens curvature, whereas any quartic terms may be thought of as applying correction for third order aberration, and so on.
Aberration is often expressed as a wave front distortion, in which case, the amount of distortion is proportional to the fourth power of the lens NA (numerical aperture) and the focal length. A good rule of thumb for substantially xe2x80x98aberration freexe2x80x99 performance is that the wave distortion must not exceed xcex/4 across the aperture. To demonstrate the lack of efficiency of a conventional lens structure, a ray tracing program was used to calculate the distortion for a typical anamorphic lens at 45xc2x0 to the vertical and horizontal planes. This lens had a refractive index of 1.87, a correction ratio (anamorphic ratio) of 2.56 and a geometric mean focal length of 1 mm.
This calculation revealed that the wave front distortion at 1.48 xcexcm is given by:
4) xcex94xcfx86=41NAg2NAy2 (in waves)
Where NAx is the horizontal numerical aperture in waves and NAy is the vertical numerical aperture. It is convenient to express equation 4) in terms of a xe2x80x98geometricalxe2x80x99 numerical aperture defined by:
5) NAg={square root over (NAxNAy)}
Thus, expressing equation 4) in terms of the geometrical NA, we have:
6) xcex94xcfx86=41NAg4
for the xcex/4 condition to be fulfilled, then:
7) 41NAg4 less than 0.25 or NAg less than 0.27.
For a typical current laser design, NAx=0.144 and NAy=0.387, giving NAg=0.24. This is just within the xe2x80x98limitxe2x80x99 defined above. However, future higher power laser chip designs will have a much higher NA in order to maximise ex-facet power. For these designs, the current anamorphic lens design will no longer be effectively aberration free. Although current lens designs can correct aberration in the horizontal and vertical planes, they are, when presented with a highly divergent elliptical beam, significantly less effective in the correction of aberration in the planes at 45xc2x0 to the horizontal and vertical planes. This reduces the energy that can be coupled into a circular cross-section fibre and partially negates the advantage of introducing a high power source.
An object of the invention is to overcome or at least to mitigate the above disadvantage.
A further object of the invention is to provide an improved anamorphic lens construction.
A further object of the invention is to provide an improved optical pump source for an amplifier.
According to a first aspect of the invention there is provided an anamorphic lens having first and second curved surfaces having mutually perpendicular planes of symmetry, wherein each said surface is defined from a generator polynomial including cross terms in first and second independent variables so as to correct aberration of light from a widely divergent source.
The generator polynomial comprises a function defining the curvature of the surface summed with a plurality of polynomial terms providing correction of aberration.
In a preferred embodiment, each lens surface is defined by a polynomial expression of the form,   z  =                    x        2                              R          x                +                                            R              x              2                        -                                          (                                  1                  +                                      k                    x                                                  )                            ⁢                              xe2x80x83                            ⁢                              y                2                                                          +                  y        2                              R          y                +                                            R              y              2                        -                                          (                                  1                  +                                      k                    y                                                  )                            ⁢                              xe2x80x83                            ⁢                              y                2                                                          +                  α        10            ⁢              xe2x80x83            ⁢              x        2              +                  α        01            ⁢              xe2x80x83            ⁢              y        2              +                  α        20            ⁢              xe2x80x83            ⁢              x        4              +                  α        11            ⁢              xe2x80x83            ⁢              x        2            ⁢              xe2x80x83            ⁢              y        2              +                  α        02            ⁢              xe2x80x83            ⁢              y        4              +                  α        30            ⁢              xe2x80x83            ⁢              x        6              +                  α        21            ⁢              xe2x80x83            ⁢              x        4            ⁢              xe2x80x83            ⁢              y        2              +                  α        12            ⁢              xe2x80x83            ⁢              x        2            ⁢              xe2x80x83            ⁢              y        4              +                  α        03            ⁢              xe2x80x83            ⁢              y        6            ⁢      …      
The coefficients in this polynomial may be determined by the use of ray tracing software.
According to another aspect of the invention there is provided a method of fabricating an anamorphic lens having first and second cylindrical surfaces having mutually perpendicular planes of symmetry, wherein each said surface is defined from a generator polynomial including cross terms in first and second independent variables so as to correct aberration of light from a widely divergent source, the method comprising: generating a trial design, and selectively adjusting coefficients in the polynomial corresponding to that trial design so as to optimise the lens design so as to provide a minimised wave front distortion.
In another aspect, the invention provides an improved anamorphic lens design for the efficient coupling of laser radiation from optoelectronic devices into optical fibres and other waveguide devices. More particularly, the lens manipulates the highly elliptical beam produced by high power laser sources to produce an almost circular parallel beam suitable for efficient coupling into optical fibres and other waveguide devices. The configuration of the lens surfaces is designed to minimise aberration.
In a preferred embodiment, the anamorphic lens provides collimation of a pump laser beam in a Raman amplifier assembly e.g. for use in an optical communications system.
The lens surface may be formed by micro-machining either the lens itself or a ceramic mould employed to manufacture the lens.